radeonsi: Fix memory leak in si_set_constant_buffer.

Fixes resource leak defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

src/gallium/drivers/radeonsi/si_state.c

@@ -2523,6 +2523,7 @@ static void si_set_constant_buffer(struct pipe_context *ctx, uint shader, uint i
 
 	default:
 		R600_ERR("unsupported %d\n", shader);
+		FREE(pm4);
 	}
 
 	if (cb->buffer != &rbuffer->b.b)